Man's need to reduce friction dates back to ancient times. As far back as 1400 B.C., both mutton fat and beef fat (tallow) were used in attempting to reduce axle friction in chariots.
Until the mid-1800s, lubricants continued to be primarily mutton and beef fats, with certain types of vegetable oils playing minor roles. In 1859, however, Colonel Drake drilled his first oil well. Since that time most lubricants, including greases, have been based on petroleum ("mineral") oil, although synthetic oil based lubricants are used for special applications.
In the NLGI Lubricating Grease Guide, C 1987, available from the National Lubricating Grease Institute, Kansas City, Mo., USA, is a detailed discussion of greases, including various types of thickeners. Such thickeners include metal soap, complex metal salt-metal soap and non-soap thickened greases.
Metal soap thickened greases have provided exemplary performance. However, under certain conditions an increased dropping point as measured by ASTM Procedure D-2265 is required.
One way to increase the dropping point of base greases is to convert a simple metal soap grease to a complex grease by incorporating therein certain acids, typically carboxylic acids such as acetic acid, alpha-omega-dicarboxylic acids and certain aromatic acids. This process necessarily consumes considerable time resulting in reduced production.
Doner et al, in a series of U.S. Patents, specifically, U.S. Pat. Nos.
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teaches increased thickening of metal salt thickened base greases is obtained employing a wide variety of boron-containing compounds. Other additives contemplated for use with boron-containing compounds are phosphorus- and sulfur-containing materials, particularly zinc dithiophosphates.
Reaction products of O,O-dihydrocarbylphosphorodithioic acids with epoxides are described by Asseff in U.S. Pat. No. 3,341,633. These products are described as gear lubricant additives and as intermediates for preparing lubricant additives.
U.S. Pat. No. 3,197,405 (LeSuer) describes phosphorus and nitrogen containing compositions prepared by forming an acidic intermediate by the reaction of a hydroxy substituted triester of a phosphorothioic acid with an inorganic phosphorus reagent and neutralizing a substantial portion of said acidic intermediate with an amine. These compositions are described as lubricant additives.
U.S. Pat. No. 4,410,435 (Naka et al) teaches a lithium complex grease containing a base oil, a fatty acid having 12-24 carbon atoms, a dicarboxylic acid having 4-12 carbon atoms and/or a dicarboxylic acid ester and lithium hydroxide thickened with a phosphate ester and/or a phosphite ester.